Optical scanning device with coma correction for improved focus tracking signal

ABSTRACT

An optical scanning device ( 1 ) includes: a radiation source ( 4 ) for supplying a radiation beam ( 8 ); a lens system ( 5 ) for transforming the radiation beam to a scanning spot ( 14 ); and a detection system ( 6 ) that includes a quadrant detector ( 20 ), an astigmatism generating element ( 18 ) for generating a first amount of coma (W 31   a ) so as to transform the radiation beam to a first astigmatic radiation beam ( 21 ), and a coma correcting element ( 19 ) for generating a second amount of coma (W 31   b ) so as to compensate the first amount of coma. The coma correcting element includes a correction surface ( 19 A) having a shape defined by a function “H(r, θ)” that includes the term “A.r 3 .cos(θ)” wherein: “H” is the position of the correction surface along the optical axis of the lens system, “r” and “θ” designate polar coordinates, and “A” designates a first constant dependent on the second amount of coma.

The invention relates to an optical scanning device for scanning aninformation layer of an optical record carrier, the device including:

-   -   a radiation source for supplying a radiation beam,    -   a lens system for transforming said radiation beam to a scanning        spot in the position of said information layer, this system        having an optical axis, and    -   a detection system including:        -   an astigmatism generating element for transforming said            radiation beam to a first astigmatic radiation beam, this            element further generating a first amount of coma so that            the first astigmatic radiation beam includes coma            aberration,        -   a coma correcting element for generating a second amount of            coma so as to compensate said first amount of coma, thereby            transforming said first astigmatic radiation beam to a            second astigmatic radiation beam that is substantially free            from coma aberration, and        -   a quadrant detector for transforming said second astigmatic            radiation beam to an electrical signal.

“Scanning an information layer” refers to scanning by a radiation beamfor: reading information from the information layer (“reading mode”),writing information in the information layer (“writing mode”), and/orerasing information from the information layer.

Generally speaking, in a conventional optical scanning device of thetype described in the opening paragraph, a “focus error signal” is usedfor maintaining the scanning spot in focus in the information layerwhich is to be scanned. This signal is commonly formed from thewell-known “astigmatic method” which is known from, inter alia, G.Bouwhuis, J. Braat, A. Huijser et al, “Principles of Optical DiscSystems,” 75–80 (Adam Hilger 1985) (ISBN 0-85274-785-3). The astigmaticmethod is based upon the voluntary introduction of an opticalaberration, astigmatism, in the optical path of the radiation beam.Typically, a plane parallel plate that is tilted with an angle of 45degrees with respect to the optical axis is used as the astigmatismgenerating element: when the radiation beam traverses this element,astigmatism is generated.

Furthermore, a “radial-tracking error signal” is used for maintainingthe scanning spot on track. This signal is commonly formed from thewell-known “radial push-pull method” which is known from, inter alia, G.Bouwhuis, J. Braat, A. Huijser et al, “Principles of Optical DiscSystems,” 70–73 (Adam Hilger 1985) (ISBN 0-85274-785-3).

A problem encountered with such a conventional optical scanning deviceresides in that the plane parallel plate generates, apart fromastigmatism, an additional aberration, coma. Coma can be expressed inthe form of the Seidel coefficient W₃₁. The following equation gives the“root-mean-square” W_(31rms), normalized with respect to the wavelengthλ, of the coefficient W₃₁:

$\begin{matrix}{W_{31{rms}} = {\frac{{- {n^{2}\left( {n^{2} - 1} \right)}} \cdot {\sin(\alpha)} \cdot {\cos(\alpha)}}{2 \cdot \left( {n^{2} - {\sin^{2}\alpha}} \right)^{\frac{5}{2}}} \cdot \frac{d}{\lambda} \cdot \frac{{NA}^{3}}{\sqrt{72}}}} & (1)\end{matrix}$wherein “d” designates the thickness of the plane parallel plate, “n”designates the refractive index of the plane parallel plate, “α”designates the angle of the plane parallel plate with the optical axis(preferably 45 degrees), and “NA” designates the numerical aperture ofthe radiation beam that is incident to the plane parallel plate. Forfurther information, see e.g. M. Born and E. Wolf, “Principles ofOptics,” 469–470 (Pergamon Press) (ISBN 0-08-026482-4).

The presence of such a coma aberration is not desired since it affectsthe focus tracking signal, because the spot on the center of thequadrant-detector is not symmetrical due to the amount of coma generatedby the plane parallel plate.

A first solution to this problem consists in reducing the thickness d ofthe plane parallel plate, since the generated amount of coma (that maybe expressed in the form of W_(31rms) as given in Equation (1)) variesproportionally with the thickness d.

However, the first solution has the following drawbacks. Firstly, thethickness of the plane parallel plate also affects the amount ofastigmatism generated by the plate, since the plate generates an amountof astigmatism, expressed in the form of the Seidel coefficient W₂₂. Thefollowing equation gives the “root-mean-square” W_(22rms), normalizedwith respect to the wavelength λ, of the coefficient W₂₂:

$\begin{matrix}{W_{22{rms}} = {\frac{\left( {n^{2} - 1} \right) \cdot {\sin^{2}(\alpha)}}{2 \cdot \left( {n^{2} - {\sin^{2}\alpha}} \right)^{\frac{3}{2}}} \cdot \frac{d}{\lambda} \cdot \frac{{NA}^{3}}{\sqrt{24}}}} & (2)\end{matrix}$wherein “d,” “n,” “α” and “NA” are the same as those defined in Equation(1). Secondly, even a thin plate generates a substantial amount of coma.For instance, calculations show that a plane parallel plate with a 1.1mm thickness generates an amount of coma that equals 71 mλrms in thecase where NA=0.135, α=45 degrees, n=1.51. Thirdly, the thickness of theplate is preferably larger than 1 mm because of mechanical constraints,especially bending limitations on the carrier.

A second solution consists in reducing the numerical aperture NA of theradiation beam that is incident to the plane parallel plate, since thegenerated amount of coma (that may be expressed by W_(31rms) as given inEquation (1)) varies with the numerical aperture NA. However, the secondsolution has the drawback that it also reduces the laser powerassociated with the radiation source, which also depends on thenumerical aperture NA.

Other solutions to the aforementioned problem exist in the state of theart.

U.S. Pat. No. 4,709,139 describes an optical scanning device of the typedescribed in the opening paragraph, wherein the coma correcting elementis formed by a plane parallel plate that is tilted oppositely to theastigmatism generating element in respect of the optical axis of thelens system. As a result, the astigmatism generating element and thecoma correcting element generate equal amounts of astigmatism that aredirected in the same direction, as well as equal amounts of coma thatare directed in opposite directions: therefore, the amount of comagenerated by the astigmatism generating element is eliminated by theamount of coma generated by the coma correcting element.

However, the device as described in U.S. Pat. No. 4,209,139 has thedrawback that it needs an additional plane parallel plate, taking upspace, generating an extra amount of astigmatism, and preventing focusadjustment of the spot with the quadrant detector.

U.S. Pat. No. 5,496,993 describes an optical scanning device of the typedescribed in the opening paragraph, wherein the astigmatism generatingelement is formed by a plane parallel plate and the coma correctingelement is formed by a wedge-shaped optical element. The wedge-shapedelement has an entrance surface that is inclined in a directiondiametrically opposite to the direction of inclination of the plate,thereby producing an amount of coma that is directed diametricallyopposite to the direction of the amount of coma generated by the plate.In other words, the amount of coma generated by the wedge-shaped elementcompensates the amount of coma generated by the plate.

However, the device as described in U.S. Pat. No. 5,496,993 hasdrawbacks. In particular, the coma correcting element generates, apartfrom coma, an extra amount of astigmatism.

An object of the invention is to provide an optical scanning device thatremedies the aforementioned disadvantages.

In accordance with the invention, these objects are achieved by anoptical scanning device as described in the opening paragraph, which ischaracterized said coma correcting element includes a correction surfacehaving a shape defined by a function “H(r, θ)” that includes the term“A.r³.cos(θ)” wherein: “H” is the position of the correction surfacealong the optical axis of the lens system, “r” and “θ” designate polarcoordinates in a cross-section of the first astigmatic radiation beam,and “A” designates a first constant dependent on the amount of comagenerated by the coma correction element.

In a preferred embodiment of the optical scanning device according tothe invention, the function “H(r,θ)” is defined by:H(r,θ)=A.r ³.cos(θ)+B.r+C.r ².cos²(θ−θo)wherein “B,” “C” and “θo” designate a second constant, a third constantand a fourth constant, respectively.

An advantage of such a coma correcting element is that it generates thesecond amount of coma for compensating the first amount of coma, withoutgenerating an additional amount of astigmatism and without affecting theamount of astigmatism generated by the astigmatism generating element.

Another advantage of such a coma correcting element is that it reducesthe radial-to-focus cross-talk, since the cross-section of the secondastigmatic radiation beam is no longer deformed by coma aberration.

Another advantage of such a coma correcting element is that the secondastigmatic radiation beam is not tilted, thereby avoiding a shift of theposition of the quadrant detector and consequently avoiding an increasein the free working distance.

An advantage of such a coma correcting element is that it may be formedas the entrance surface of a servo lens; the exit surface of the servolens may be advantageously used for forming, for instance, a negativespherical lens. This advantageously results in allowing focus adjustmentof the second astigmatic radiation beam with the quadrant detector, bymoving the servo lens in the direction of the optical axis of the lenssystem and, in the case where the servo lens is not formed by anaspherical lens, by moving the quadrant detector.

Another advantage of such a coma correcting element is that it may beused for focus adjustment of the spot with the quadrant detector.

The objects, advantages and features of the invention will be apparentfrom the following, more detailed description of the invention, asillustrated in the accompanying drawings, in which:

FIG. 1 is a schematic illustration of components of an optical scanningdevice according to one embodiment of the invention,

FIG. 2 is a schematic illustration of components of the detection systemshown in FIG. 1,

FIG. 3, FIG. 4 and FIG. 5 show the cross-sectional views of first,second and third embodiments of the coma correcting element shown inFIG. 2, respectively,

FIGS. 6A and 6B are the schematic representations of cross-sections ofthe second astigmatic radiation beam on the quadrant detector, with andwithout correction according to the invention, respectively, and

FIGS. 7A and 7B show the focus S-curves measured with respect to theradiation beam that is incident to the quadrant-detector, with andwithout correction according to the invention, respectively.

FIG. 1 is a schematic illustration of the optical components of anoptical scanning device 1 according to the invention for scanning aninformation layer 2 of an optical record carrier 3.

By way of illustration, the optical record carrier 3 includes atransparent layer 60 on one side of which the information layer 2 isarranged. The side of the information layer facing away from thetransparent layer is protected from environmental influences by aprotective layer 61. The transparent layer acts as a substrate for theoptical record carrier by providing mechanical support for theinformation layer. Alternatively, the transparent layer may have thesole function of protecting the information layer, while the mechanicalsupport is provided by a layer on the other side of the informationlayer, for instance by the protection layer or by an additionalinformation layer and transparent layer connected to the uppermostinformation layer. The information layer 2 is a surface of the carrier 3containing tracks. A “track” is a path to be followed by a focusedradiation on which optically-readable marks that represent informationare arranged. The marks may be, e.g., in the form of pits or areas witha reflection coefficient or a direction of magnetization different fromthe surroundings. In the case where the optical record carrier 3 has theshape of a disc, the following is defined with respect to a given track:the “radial direction” is the direction between the track and the centerof the disc and the “tangential direction” is the direction that istangential to the track and perpendicular to the “radial direction”.

During scanning, the record carrier 3 rotates on a spindle (not shown inFIG. 1) and the information layer 2 is then scanned through thetransparent layer 60.

As shown in FIG. 1, the optical scanning device 1 includes a radiationsource 4, a lens system 5 having an optical axis OO′, a beam splitter 18and a detection system 6. The optical scanning device 1 preferablyfurther includes a servocircuit 6A, a focus actuator 6B, a radialactuator 6D, and an information processing unit 6C for error correction.

The radiation source 4 is arranged for supplying a radiation beam 8.Preferably, the radiation source 4 includes at least one semiconductorlaser that emits the radiation beam 8 at a selected wavelength λ. Forinstance, in the case where the optical record carrier 3 is of a DVDformat, the wavelength λ of the radiation beam 8 is between 620 and 700nm and, preferably, equals 660 nm. More preferably, the optical scanningdevice 1 further includes a grating structure 9 for forming first andsecond satellite radiation beams 10 and 11 as the −1 and +1 orderdiffracted beams from the central radiation beam, that is, the radiationbeam 8.

The beam splitter 18 is arranged for reflecting the radiation beam 8 (aswell as the satellite radiation beams 10 and 11) toward the lens system5. Preferably, the beam splitter 18 is formed by a plane parallel platethat is tilted with respect to the optical axis OO′ so as to form anangle α with respect to this axis. Preferably, the angle α equals to 45degrees. Notably, the plane parallel plate is used, apart from itsfunction of beam splitter, for generating astigmatism as describedbelow.

The lens system 5 is arranged for transforming the radiation beam 8 (aswell as the satellite radiation beams 10 and 11) to a focused radiationbeam 13 so as to form a scanning spot 14 in the position of theinformation layer 2. The lens system 5 includes a first objective lens15; it preferably further includes a collimator lens 7 and a secondobjective lens 16. Preferably, the second objective lens 16 is usedtogether with the first objective lens 15 in the case where thenumerical aperture of the radiation beam 8 approximately equals 0.85,while only the first objective lens 15 is used in the case where thenumerical aperture of the radiation beam 8 is comprised between 0.45 and0.65.

The collimator lens 7 is arranged for transforming the radiation beam 8(as well as the satellite radiation beams 10 and 11) into asubstantially collimated beam 12.

The first objective lens 15 is arranged for transforming the collimatedradiation beam 12 to a converging radiation beam 17. Furthermore, theobjective lens 15 is preferably aspherical.

The second objective lens 16 is arranged for transforming the convergingradiation beam 17 to the focused radiation beam 13. It may be formed bya plano-convex lens having a convex surface 15 a that faces theobjective lens 15 and a flat surface 15 b that faces the position of theinformation layer 2. Notably, the objective lens 16 forms, incooperation with the objective lens 15, a doublet-lens system thatadvantageously has a larger tolerance in mutual position of the opticalelements than the single-lens system. Furthermore, the objective lens 16is preferably aspherical.

By way of illustration, in the case where the optical record carrier 3is of a DVD format, the selected numerical aperture of the focused beam13 approximately equals to 0.60 for the “reading mode” and to 0.65 forthe “writing mode.”

Thus, during scanning, the focused radiation beam 13 reflects on theinformation layer 2, thereby forming a reflected beam which returns onthe optical path of the forward converging beam 17. The lens system 5transforms the reflected radiation beam to a first converging reflectedradiation beam 63. Finally, the beam splitter 18 separates the forwardradiation beam 8 from the reflected radiation beam 17 by transmitting atleast part of the reflected radiation beam 63 towards the detectionsystem 6, in the form of a radiation beam 21.

The detection system 6 is arranged for capturing the radiation beam 21and the satellite radiation beams and converting them to one or moreelectrical signal. One of the signals is an information signal I_(data),the value of which represents the information scanned on the informationlayer 2. The information signal I_(data) is processed by the informationprocessing unit 6C for error correction. Other signals from thedetection system 6 are a focus error signal I_(focus) and a radialtracking error signal I_(radial). The signal I_(focus) represents theaxial difference in height (along the optical axis of the lens system 5)between the scanning spot 14 and the position of the information layer2. Preferably, this signal is formed from the “astigmatic method” asdescribed above. The radial tracking error signal I_(radial) representsthe distance in the plane of the information layer 2 between thescanning spot 14 and the center of a track in the information layer 2 tobe followed by the scanning spot 14. Preferably, this signal is formedfrom the “radial push-pull method” as described above.

The servocircuit 6A is arranged for, in response to the signalsI_(focus) and I_(radial), providing servo control signals I_(control)for controlling the focus actuator 6B and the radial actuator 6D,respectively. The focus actuator 6B controls the position of theobjective lenses 15 and 16 along the optical axis of the lens system 5,thereby controlling the position of the scanning spot 14 such that itcoincides substantially with the plane of the information layer 2. Theradial actuator 6D controls the position of the objective lenses 15 and16 in a direction perpendicular to the optical axis of the lens system5, thereby controlling the radial position of the scanning spot 14 suchthat it coincides substantially with the center line of the track to befollowed in the information layer 2.

FIG. 2 is a schematic illustration of the detection system 6 of FIG. 1;it shows in detail that the detection system 6 includes an astigmatismgenerating element (preferably formed by the beam splitter 18), a comacorrecting element 19 and a quadrant detector 20. As a matter of purelyarbitrary choice, the reference “Z-axis” designates the direction of theoptical axis OO′ of the lens system 5, and the references “X-axis” and“Y-axis” designate the two directions of the quadrant detector 12 thatcorresponds to the radial and tangential directions, respectively.

The astigmatism generating element 18 is arranged for transforming theradiation beam 8 to a first astigmatic radiation beam 21. Furthermore,the astigmatism generating element 18 generates a first amount of comaso that the astigmatism radiation beam 21 includes coma. In thepreferred case where the astigmatism generating element is formed by theplane parallel plate that forms the beam splitter 18, the plate formsthe angle α with respect to the Z-axis, that preferably equals 45degrees, as described above. In this preferred case, said first amountof coma may be expressed in the form of the Siedel coefficient W_(31rms)^(a) similarly to that given by Equation (1).

The coma correcting element 19 is arranged for generating a secondamount of coma so as to compensate said first amount of coma (that is,in the preferred case, W_(31rms) ^(a)), thereby transforming theastigmatic radiation beam 21 to a second astigmatic radiation beam 22that is substantially free from coma aberration (the term“substantially” being explained below). In the following, said secondamount of coma may be expressed in the form of the root-mean-squareW_(31rms) ^(b) of the Seidel coefficient W₃₁ ^(b). Furthermore, the comacorrecting element 19 is formed by a correction surface 19A having ashape defined by a function “H(r, θ)” that includes the term“A.r³.cos(θ)” wherein: “H” is the position of the correction surfacealong the optical axis of the lens system, “r” and “θ” designate polarcoordinates in a cross-section of the astigmatic radiation beam 21, and“A” designates a first constant dependent on the amount of comagenerated by the coma correction element 19 (that is, on the normalizedroot-mean-square W_(31rms) ^(b) as described below in further detail).

In a preferred embodiment of the optical scanning device 1, the function“H(r,θ)” is defined by:H(r,θ)=A.r ³.cos(θ)+B.r+C.r ².cos²(θ−θo)   (3)wherein: “H,” “r,” “θ,” are those defined above, and “B,” “C” and “θo”designate a second constant, a third constant and a fourth constant,respectively.

In a preferred embodiment of the optical scanning device 1, thecorrection surface 19A is formed as the entrance surface of a lens 19′,thereby the exit surface 19B of the lens 19′ to provide an additionaloptical function. For instance, the exit surface 19B may be sphericallycurved in order to form a spherical lens. Three embodiments of the lens19′ that include three embodiments of the correction surface 19A aredescribed in detail below.

The quadrant detector 20 is arranged for converting the astigmaticradiation beam 22 to the signals I_(data), I_(focus) and I_(radial). Inorder to generate the signal I_(focus) according to the “astigmaticmethod,” the quadrant detector 20 includes: (a) four firstradiation-sensitive detection elements C1 through C4 (as shown in FIG.2) for providing four detection signals I_(C1), I_(C2), I_(C3) andI_(C4), respectively, and (b) a first electronic circuit for, inresponse to the signals I_(C1) through I_(C4), providing the signalsI_(data) and I_(focus). In order to generate the radial-tracking errorsignal I_(radial) according to the well-known “radial push-pull method,”the detection system 39 includes second radiation-sensitive detectionelements and a second electronic circuit for, in response to the outputsignals of these detection elements, providing the signal I_(radial).

Three embodiments of the lens 19′ that include three embodiments of thecorrection surface 19A are now described in detail. FIGS. 3 through 5show the cross-sectional views of first, second and third embodiments ofthe lens 19′ that include first second and third embodiments of thecorrection surface 19A, respectively. The reference numerals 19′1, 19′2and 19′3 designate the first, second and third embodiments of the lens19′, respectively, the reference numerals 19A1, 19A2 and 19A3 designatethe first, second and third embodiments of the correction surface 19A,respectively, and the reference numerals 19B1, 19B2 and 19B3 designatethe first, second and third embodiments of the exit surface of thelenses 19′1, 19′2 and 19′3, respectively.

With reference to FIG. 3, in the case where A≠0, B=0 and C=0 in Equation(3), the correction surface 19A1 corresponds to the following equation:H1(r,θ)=A1.r ³.cos(θ)   (4)wherein “A1” corresponds to the constant “A” defined with respect toEquation (3). As previously stated, A1 depends on W_(31rms) ^(b); morespecifically, it is given by the following equation:

$\begin{matrix}{{A1} = \frac{W_{31{rms}}^{b} \cdot \lambda \cdot \sqrt{72}}{L^{3} \cdot {NA}^{3} \cdot \left( {n_{lens} - 1} \right)}} & (5)\end{matrix}$wherein: “W_(31rms) ^(b)” is the “root-mean-square” value associatedwith the amount of coma W₃₁ ^(b) generated by the correction surface19A1; “λ” is the wavelength of the astigmatic radiation beam 22; “L” isthe distance from the correction surface 19A1 to object of the sphericallens 19B1 (as shown in FIG. 3); “NA” is the numerical aperture of theastigmatic radiation beam 22; and “n_(lens)” is the refractive index ofthe lens 19′1. Furthermore, in order to compensate the amount of comagenerated by the plane parallel plate 18, the value W_(31rms) ^(b) inEquation (5) is to ideally equal the “root-mean-square” value W_(31rms)^(a) given by Equation (1).

By way of illustration only, if d=1.1 mm, NA=0.135, α=45 degrees andn=1.51 in Equation (1), the value W_(31rms) ^(a) equals to 71 mλrms.And, if n_(lens)=1.57, NA=0.16, λ=790 nm, L=2.8 mm, an ideal valueA1_(ideal) of the constant A1 is finally known from Equation (5) whereW_(31rms) ^(b) ideally equals 71 mλrms. In practice, the ideal valueA1_(ideal) cannot be obtained and the actual value A1_(actual) of theparameter results in compensating the first amount of coma W₃₁ ^(a) soas to form the second astigmatic radiation beam 22 that is substantiallyfree from coma. For instance, if A1_(actual)=0.1 mm⁻², the resultingvalue W_(31rms) ^(b) equals to 71 mλrms. In other words, a difference of5 mλrms between the ideal and actual values may be tolerate; in thedescription, “substantially free from coma” means that the“root-square-mean” value of the resulting amount of coma in theastigmatic radiation beam 22 is less than 10 mλrms.

With reference to FIG. 4, in the case where A≠0, B≠0 and C=0 in Equation(3), the correction surface 19A2 corresponds to the following equation:H2(r,θ)=A2.r ³.cos(θ)+B2.rwherein “A2” and “B2” correspond to the constants “A” and “B,”respectively, defined with respect to Equation (3). In the case wherethe astigmatism generating element 18 is formed by a plate, the constantA2 may be calculated as described with respect to FIG. 3. The constantB2 represents a constant tilt of the correction surface 19A2 forcompensating the average tilt of the correction surface curved by theterm “A.r³.cos(θ)” in the Y-direction; it typically is less than onedegree. Notably, the constant tilt has no significant effect on the comacorrection; however, it makes the correction surface 19A2 advantageouswith respect to the surface 19A1, in terms of mould making. Anotheradvantage of forming such a cylindrical surface is that it generates asecond amount of astigmatism in addition to the amount generated by theplane parallel plate.

With reference to FIG. 5, in the case where A≠0, B≠0, C≠0 and θo=0 inEquation (3), the correcting surface 19A3 corresponds to the followingequation:H3(r,θ)=A3.r ³ cos θ+B3.r+C3.r ².cos²(θ)wherein “A3,” “B3” and “C3” correspond to the constants “A,” “B” and“C”, respectively, defined with respect to Equation (3). Notably, the“C3.r²cos²(θ)” term represents a cylindrical surface that generates anadditional amount of astigmatism that is added to the amount ofastigmatism generated by the plate 18, i.e. W₂₂ as expressed in Equation(2); the coefficient C3 then may be expressed as follows:

${C3} \approx \frac{1}{2 \cdot {Rcyl}}$wherein “Rcyl” is the cylinder radius associated with the cylindricalsurface.

Alternatively, in the case where C≠0 and θo≠0, the azimuth of thecorresponding cylindrical surface forms an angle θo with respect to theY-direction. This advantageously results in rotating the focal lines ofthe astigmatic radiation beam 21 that emerges from the plane parallelplate 18.

As a matter of illustration, the operation of the optical scanningdevice 1 is described below—in particular, the effect of the comacorrecting element 19 on the radiation beam 21. FIGS. 6A and 6B areschematic representations of the cross-sections of the radiation beam 22on the quadrant detector 20, with and without correction according tothe invention, respectively. And FIGS. 7A and 7B show the focus S-curves(that have been produced by simulation) with respect to the radiationbeam 22 that is incident to the quadrant-detector, with and withoutcorrection according to the invention, respectively.

FIGS. 6A and 6B show dots that represent the intersection of rays of theradiation beam 22 with the plane of the quadrant detector 20. Withreference to FIG. 6A, the astigmatic radiation beam 22, with comacorrection according to the invention, is substantially symmetrical withrespect to the center of the beam. With reference to FIG. 6B, theastigmatic radiation beam 22, without correction according to theinvention, is affected by the first amount of coma and is consequentlysymmetrical with respect to the center of the beam.

In relation to FIG. 7A and FIG. 7B, the normalized value FE of thefocus-S curves (with respect to the maximum value of the signalI_(focus)) are derived from a measurement of the detection signalsI_(C1) through I_(C4) according to the following equation:

${FE} = \frac{I_{c1} + I_{c3} - I_{c2} - I_{c4}}{I_{c1} + I_{c3} + I_{c2} + I_{c4}}$

With reference to FIG. 7A, the focus-S curve 61 associated with theastigmatic radiation beam 22, with correction according to the inventionis substantially symmetrical with respect to the point that correspondsto FE=0.

With reference to FIG. 7B, the focus-S curve 62 associated with theastigmatic radiation beam 22, without correction according to theinvention, has a peak 63 that is affected by the presence of coma in theradiation beam 22. The skilled person notes that a disadvantage of thepresence of such a peak is that the optical scanning device isparticularly sensitive to chocks.

An advantage of such a coma correcting element is that, when adjustingthe position of the lens 19′ with respect to the quadrant-detector 20for focus adjustment purposes, it results in an insignificant change inthe amount of coma generated by the correcting element. For instance,calculations have shown that a change of 0.2 mm in the position of thelens 19′ along the axis OO′ results in a change of 5 mλrms in thegenerated amount of coma which is considered not to be significant withrespect to 71 mλrms.

It is to be appreciated that numerous variations and modifications maybe employed in relation to the embodiments described above, withoutdeparting from the scope of the invention which is defined in theappended claims.

As an alternative, the coma correcting element may be combined with anyastigmatism generating element other than a plane parallel plate, thatfurther generates coma aberration.

As an alternative, the optical scanning device may be of the typecapable to performing simultaneous multi-track scanning. This results inimproving data read-out in the “reading mode” and/or write speed in the“writing mode” as described, for example, in U.S. Pat. No. 4,449,212.The description of the multi-tracking arrangement according to U.S. Pat.No. 4,449,212 is incorporated herein by reference.

1. An optical scanning device (1) for scanning an information layer (2)of an optical record carrier (3), the device including: a radiationsource (4) for supplying a radiation beam (8), a lens system (5) fortransforming said radiation beam to a scanning spot (14) in the positionof said information layer, this system having an optical axis (OO′), anda detection system (6) including: an astigmatism generating element (18)for transforming said radiation beam to a first astigmatic radiationbeam (21), this element further generating a first amount of coma (W₃₁^(a)) so that the first astigmatic radiation beam includes comaaberration, a coma correcting element (19) for generating a secondamount of coma (W₃₁ ^(b)) so as to compensate said first amount of coma,thereby transforming said first astigmatic radiation beam to a secondastigmatic radiation beam (22) that is substantially free from comaaberration, and a quadrant detector (20) for transforming said secondastigmatic radiation beam to an electrical signal, characterized in thatsaid coma correcting element includes a correction surface (19A) havinga shape defined by a function H(r,θ) that includes the term A.r³.cos(θ)wherein: “H” is the position of said correction surface along theoptical axis of said lens system, “r” and “θ” designate polarcoordinates in a cross-section of said first astigmatic radiation beam,and “A” designates a first constant dependent on said second amount ofcoma.
 2. The optical scanning device (1) as claimed in claim 1,characterized said function “H(r,θ)” is defined by:H(r,θ)=A.r ³.cos(θ)+B.r+C.r ².cos²(θ−θo) wherein “B,” “C” and “θo”designate a second constant, a third constant and a fourth constant,respectively.
 3. The optical scanning device (1) as claimed in claim 1,characterized in that said astigmatism generating element (18) is formedby a plane parallel plate inclined at a predetermined angle (α) withrespect to said optical axis (OO′).
 4. The optical scanning device (1)as claimed in claim 1, characterized in that said correction surface(19A) is formed as the entrance surface of a lens (19′), and in thatsaid lens has a anisotropically curved exit surface (19B).
 5. Theoptical scanning device (1) as claimed in claim 1, characterized in thatsaid detection system (6) is further arranged for providing a focuserror signal (I_(focus)) and/or a radial-tracking error signal(I_(radial)) and in that it further includes a servocircuit (6A) and anactuator (6B) responsive to said focus error signal and/or saidradial-tracking error signal for controlling the position of saidscanning spot (14) with respect to the position of said informationlayer (2) and/or of a track of said information layer which is to bescanned.
 6. The optical scanning device (1) as claimed in claim 1,characterized in that it further includes an information processing unitfor error correction (6C).